1. Field of the Invention
The present invention relates to a method of fabricating a capacitor, and more particularly to a method of fabricating a capacitor formed in a re-wiring layer of a semiconductor device or a wiring layer of a printed circuit board or the like.
2. Description of the Background Art
In recent years, a demand for miniaturization of electronic devices has been strong and, accordingly, the size of each of various electronic parts is being reduced. A capacitor is not an exception and a demand for miniaturization of capacitors is also strong.
Conventionally, a capacitor is formed in a re-wiring layer in a semiconductor device. On the other hand, a printed circuit board (PCB) is constructed by mounting a capacitor element on the surface of the board, so that its miniaturization is not sufficient. One of methods of realizing miniaturization of a capacitor is a method of providing a capacitor in a printed wiring board so as to be formed in a re-wiring layer of a semiconductor device.
One of capacitor fabricating methods in the case of forming a capacitor in a printed wiring board is a method of forming a dielectric layer by applying a paste resin containing dielectric particles by printing. Although the method is excellent with respect to the point that the dielectric layer can be formed easily, it also has a problem that it is difficult to form a dielectric layer having a high dielectric constant. The problem is caused by the fact that there is a natural limitation on the viscosity of a material applied by printing. Specifically, in the case of mixing a large amount of dielectric particles into a paste resin material to increase the dielectric constant, the viscosity of the material applied exceeds the limit viscosity of application by printing, and it becomes difficult to form an even dielectric layer. Usually, in the case of using printing to form a fine pattern about 100 xcexcm square, the maximum amount of dielectric particles which can be contained in a resin paste from the viewpoint of viscosity is about 30 vol % to 40 vol % in volume percentages or 80% by weight in a mass ratio, so that it is difficult to form a dielectric layer having a sufficiently high dielectric constant.
As another example of the method of fabricating a capacitor in the case of providing the capacitor in a printed wiring board, there is a known method of forming a capacitor by, in a manner similar to the case of forming a capacitor in a re-wiring layer of a semiconductor device, depositing an oxide layer of high dielectric constant by CVD (Chemical Vapor Deposition) or the like. FIGS. 11 to 16 are process drawings showing an example of a method of fabricating a capacitor by using CVD. With reference to the drawings, the conventional method of fabricating a capacitor by using CVD will be described in detail later.
First, a first conductive layer 102 is prepared as shown in FIG. 1. As the first conductive layer, for example, copper foil or the like is used. On the whole principal surface of first conductive layer 102, an oxide layer 103 of high dielectric constant is deposited by CVD method. Subsequently, as shown in FIG. 12, a second conductive layer 107 is formed on oxide layer 103 of high dielectric constant. In such a manner, a sandwich structure of three layers of conductive layer/oxide layer of high dielectric constant/conductive layer is obtained.
After that, as shown in FIG. 13, a resist pattern 108 is formed in a predetermined area on second conductive layer 107, and etching is performed by using resist pattern 108 as a mask. By the operations, as shown in FIG. 14, a capacitor of a desired shape in a desired size is formed. Subsequently, as shown in FIG. 15, prepregs 109 are stacked so as to sandwich the capacitor and compression-bonded by hot press, thereby forming insulating layers 106 so as to surround the capacitor as shown in FIG. 16. The prepregs are sheet members obtained by impregnating glass cloth or the like with epoxy resin or the like.
By the processes, the capacitor can be formed in a printed wiring board and the method largely contributes to miniaturization of an electronic device. However, the fabricating method has the following problems.
First, since CVD method is used to form an oxide layer of high dielectric constant, an expensive CVD system has to be introduced. The CVD system is far more expensive than other fabricating systems used to form a printed wiring board. The introduction of the CVD system extremely increases the manufacturing cost of a printed wiring board.
Second, compression bonding by hot press is necessary to form an insulating layer by using prepregs and the formation positions of the insulating layers may be deviated due to flow of the resin in the prepregs or thermal expansion by heating. The positional deviation deteriorates the yield of products, and causes increase in manufacturing cost.
As a capacitor fabricating method which does not cause such problems, there is a capacitor fabricating method disclosed in Japanese Patent Laying-Open No. 7-92695. FIGS. 17 to 21 are process drawings showing an example of the capacitor fabricating method disclosed in the publication. With reference to the drawings, the capacitor fabricating method disclosed in the publication will be described in detail later.
First, as shown in FIG. 17, a substrate 201 is prepared and a first conductive layer 202 is formed in a predetermined area on substrate 201. Subsequently, as shown in FIG. 18, a positive photosensitive resin is applied on substrate 201 so as to cover first conductive layer 202, thereby forming a positive photosensitive resin layer 203a. As the positive photosensitive resin, polysilane resin is used. To apply the positive photosensitive resin, usually, the spin coat method or the like is used. The resin is applied on the whole surface of substrate 201.
As shown in FIG. 19, a mask 210 having a predetermined pattern is prepared and fixed so that only the positive photosensitive resin in the portion on first conductive layer 202 can be exposed, and positive photosensitive resin layer 203a is irradiated with ultraviolet rays. In such a manner, only the positive photosensitive resin on the predetermined area of first conductive layer 202 is exposed, and an exposed positive photosensitive resin layer 203b is produced.
Subsequently, as shown in FIG. 20, positive photosensitive resin layers 203a and 203b are immersed in a solution 204 in which dielectric particles 205 are dispersed. At this time, positive photosensitive resin layer 203a which is not exposed is hardly impregnated with dielectric particles 205, whereas exposed positive photosensitive resin layer 203b is impregnated with a large amount of dielectric particles 205. This phenomenon occurs for the reason that a molecular chain in the positive photosensitive resin layer is cut by exposure, and small gaps are created in the positive photosensitive resin layer. As a result, dielectric particles 205 are diffused only in exposed positive photosensitive resin layer 203b, so that only this portion functions as a dielectric layer, and the remaining positive photosensitive resin layer 203a functions as an insulating layer.
Subsequently, as shown in FIG. 21, a second conductive layer 207 is formed on positive photosensitive resin layer 203b in which dielectric particles 205 are diffused. By the above operations, a sandwich structure of three layers of first conductive layer 202/positive photosensitive resin layer 203b in which dielectric particles 205 are diffused/second conductive layer 207 is obtained, and a capacitor is fabricated. Simultaneously, an insulating layer is formed by positive photosensitive resin layer 203a which is not exposed.
By performing the above processes, the capacitor can be formed in a printed wiring board without using an expensive CVD system, and miniaturization of an electronic device can be realized at low cost. Since the dielectric layer and the insulating layer are formed by selectively exposing the positive photosensitive resin preliminarily applied on the whole surface of the substrate by spin coating or the like, there is no fear of the positional deviation which occurs in the case of using prepregs, so that the yield is largely improved. Further, the exposed positive photosensitive resin layer can be impregnated with a large amount of the dielectric particles, so that the dielectric layer of high dielectric constant can be formed.
The capacitor fabricating method using the positive photosensitive resin has, however, the following problems.
First, there is the possibility that the capacitor formed by the fabricating method has poor dielectric strength. The poor dielectric strength is caused by a large difference in the coefficient of thermal expansion between the positive photosensitive resin layer in the portion where the dielectric particles are diffused (hereinafter, referred to as a diffusion layer) and the positive photosensitive resin layer in the portion where the dielectric particles are not diffused (hereinafter, referred to as a non-diffusion layer). Usually, in the processes of fabricating a printed wiring board, due to a curing process, a process of mounting to a mother board, a change with time after shipment of a product, and the like, a large thermal shock is applied into the board. In the case where such a thermal shock is applied to the printed wiring board, a large stress concentration accompanying the difference in the coefficient of thermal expansion occurs in the boundary between the diffusion layer and the non-diffusion layer, and it is feared that a crack occurs. A crack largely deteriorates the dielectric strength characteristic of the capacitor, and a serious influence is exerted on the yield and reliability.
Second, the point that the positive photosensitive resin used to form the dielectric layer and the insulating layer is very expensive can be mentioned. Usually, a cheap resin material such as epoxy resin is used as the material of the insulating layer formed in the printed wiring board. In contrast, the positive photosensitive resin typified by the polysilane resin is very expensive. Consequently, when the above fabricating method is used, a problem such that the manufacturing cost largely increases occurs.
Third, since photolithography is used to form the dielectric layer of the capacitor, a problem that the fabricating process is complicated arises. In the photolithography process in the fabricating method, a mask for selectively exposing the positive photosensitive resin layer is necessary. Since the work such as fixing of the mask becomes necessary, the work becomes complicated, and the manufacturing cost accordingly increases.
An object of the present invention is to provide a capacitor fabricating method capable of cheaply and easily fabricating a very reliable capacitor having excellent electric characteristics such as high dielectric strength.
To achieve the object, a method of fabricating a capacitor according to the present invention includes the following steps:
(a) a step of selectively forming a positive photosensitive resin layer on a first conductive layer;
(b) a step of exposing the positive photosensitive resin layer;
(c) a step of immersing the positive photosensitive resin layer in a solution in which dielectric particles are dispersed to diffuse the dielectric particles into the positive photosensitive resin layer;
(d) a step of forming an insulating resin layer so as to cover side faces of the positive photosensitive resin layer; and
(e) a step of forming a second conductive layer on the positive photosensitive resin layer.
As described above, in the fabricating method, the positive photosensitive resin is used for the dielectric layer and the insulating resin is used for the insulating layer, so that stress in the interface of the layers can be reduced. Specifically, by forming the dielectric layer and the insulating layer by using different materials, as compared with the case of making the layers of the same material, stress which causes when thermal shock is applied is reduced, and generation of cracks is prevented Since the use amount of the positive photosensitive resin material is also largely reduced, the manufacturing cost can be reduced. Further, in the fabricating method, a process using a CVD system to form a dielectric layer and a process of selectively exposing the dielectric layer are unnecessary. Thus, the manufacturing process can be further simplified and the manufacturing cost can be further reduced.
In the method of fabricating a capacitor according to the present invention, preferably, for example, the positive photosensitive resin layer is formed by applying a positive photosensitive resin by printing method.